FIG. 1 shows a conventional, fully differential amplifier stage. It includes a differential gain pair M1 and M2, load pair M3 and M4, and a current source M5. The lower limit of the common mode input voltage is given by EQU Vlow=Vdsat.sub.m5 +Vdsat.sub.m2 +Vt.sub.m2,
wherein Vdsat.sub.mx represents the saturation voltage of transistor mx and Vt.sub.mx represents the threshold voltage of transistor mx.
The upper limit of the common mode input voltage is given by EQU Vhigh=Vdd-Vdsat.sub.m4 Vt.sub.m4 +Vt.sub.m2.
Typically, V.sub.t =0.75 v and Vdsat=0.1 V. With these values, the operation of the differential amplifier ceases when Vlow=Vhigh. EQU Vdsat.sub.m5 +Vdsat.sub.m2 +Vt.sub.m2 =Vdd-Vdsat.sub.m4 -Vt.sub.m4 +Vt.sub.m2 EQU 3Vdsat+Vt.sub.m4 =Vdd=1.05
At this voltage (Vdd=1.05), the amplifier has no usable input range. When operated at 2.7 volts=Vdd, the usable input range is 0.95 v to 2.6 v or 1.65 volts. In many applications, this range may be inadequate (see item 102 in FIG. 5). The input range may be moved (translated) by the use of zero threshold devices for M1 and M2 (i.e.: Vt.sub.m2 =0.0 v), however no additional usable input range is gained (see; item 100 in FIG. 5). Note that a zero threshold device is not available to replace M3 and M4.
It would be considered an improvement in the art, if a low-voltage differential amplifier could be made, with a wide; operable voltage range.